1. Field of the Invention
The present invention relates to a method for enhancing an image outline and a circuit thereof for use in an image display apparatus such as a television receiver, a videotape recorder or a video projector. More particularly, the present invention relates to an image outline enhancing method and a circuit thereof which can be used to enhance the outline of an image displayed by a video signal in order to improve a contour of the image.
2. Description of the Prior Art
It is known that the response of a video signal processing system, such as one found in a television receiver, may be subjectively improved by increasing the slope or "steepness" of a video signal amplitude transition. The response may also be improved by generating a signal "preshoot" just before an amplitude transition, and a signal "overshoot" just after the amplitude transition, so that an image outline displayed by the video signal is improved.
FIG. 1 is a block diagram for showing a conventional circuit for enhancing an image outline.
In FIG. 1, the circuit includes a first delay 121, a second delay 122, a third delay 123, a first adder 131, a second adder 132, a subtracter 141, and a 1/2-amplifier 151.
First and second delay sections 121 and 122 delay video signals which are continuously inputted through an input terminal 101 for a predetermined period T, respectively. The delayed signals, which are produced by first and second delay 121 and 122, are respectively outputted to first adder 131 and subtracter 151. Third delay 123 delays the delayed signals from second delay 122 for the predetermined period in order to generate a video signal delayed for period 2T. Second adder 132 adds the 2T period delayed video signal to a current video signal through input terminal 101 so that second adder 132 generates continuously added video signals, and second adder 132 provides the added video signals to 1/2-amplifier 151. 1/2-amplifier 151 1/2-amplifies the added video signals to generate 1/2-amplified signals. The 1/2-amplified signals generated by 1/2-amplifier 151 are provided to subtracter 141. Subtracter 141 subtracts the 1/2-amplified signals from the T period delayed video signals in order to generate subtracted signals, which are outputted to first adder 131 as peaking signals. First adder 131 adds the peaking signals from subtracter 141 to the delayed video signal from first delay 121, so that first adder 131 generates peaked video signals.
According to the image outline enhancing circuit 100, the circuit derives the peaking signals from video signals which are continuously inputted through the input terminal 101, and adds the peaking signals to the video signals for peaking video signals, so that the circuit enhances image outlines to be formed by the video signals.
However, in the image outline enhancing circuit 100; when an amplitude transition of a video signal occurs for a period, the generation of the preshoot before an amplitude transition and overshoot after an amplitude transition causes the amplitude transition to occur for a longer period than that of the original amplitude transition.
In order to improve an image outline without introducing the preshoot and overshoot, one example of an image outline enhancing circuit is disclosed in U.S. Pat. No. 5,247,361 (issued to Yosuke Izawa on Sep. 21, 1993). The circuit suggested by the above U.S. Patent improves an outline of an image formed by a video signal having a rising and falling slope portions and a flat portion. The circuit includes an enhancing signal producing circuit for producing an enhancing signal having a first, a second, and a third enhancing portions, wavelengths of which are the same as those of three portions of the video signal, respectively. A delay circuit is provided for delaying the video signal to be synchronized with the enhancing signal. A subtracter is further provided for subtracting the enhancing signal from the synchronized video signal, such that three enhancing portions of the enhancing signal are exactly superimposed on the rising slope, the falling slope and the flat portions, respectively. Therefore, the circuit enhances an outline of an image to be displayed according to a video signal in an image display system without introducing a preshoot and an overshoot.
However, though the circuit improves a quality of a reproduced image by enhancing an outline of the reproduced image without introducing the preshoot and overshoot, the circuit has a problem that an original video signal is modified because the amplitude transition time is shortened, that is, a length of the flat portion length of the video signal is longer than that of the original flat portion thereof. And, this modification of the video signal may cause an artificial contour compensation at edges of the image reproduced by the modified video signal.
For the foregoing reasons, there is need for an image outline enhancing method and a circuit thereof that can enhance an outline of an image to be reproduced according to a video signal without modifying an amplitude transition time as well as introducing the preshoot and overshoot.